Method of forming a single common laser resonator cavity and an optically segmented composite gain medium

ABSTRACT

A compact solid state laser that generates multiple wavelengths and multiple beams that are parallel, i.e., bore-sighted relative to each other, is disclosed. Each of the multiple laser beams can be at a different wavelength, pulse energy, pulse length, repetition rate and average power. Each of the laser beams can be turned on or off independently. The laser is comprised of an optically segmented gain section, common laser resonator with common surface segmented cavity mirrors, optically segmented pump laser, and different intra-cavity elements in each laser segment.

REFERENCE TO RELATED APPLICATIONS

This is a divisional patent application of copending application Ser.No. 13/053,422 filed Mar. 22, 2011, entitled “Compact Multi-Wavelengthand Multi-Beam Laser.” The aforementioned application is herebyincorporated herein by reference.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, sold,imported, and/or licensed by or for the Government of the United Statesof America.

FIELD OF THE INVENTION

This invention relates in general to lasers, and more particularly, tomulti-wavelength and multi-beam lasers.

BACKGROUND OF THE INVENTION

Specific laser applications such as laser designation, laserrange-finding, laser illumination, and laser marking and pulsedilluminators for active imaging require separate laser devices, eachoptimized for the corresponding use. Each laser device typically comeswith its own housing, electronics and power supply/batteries. Although amulti-wavelength, multipurpose laser could be constructed by integratingthese laser devices into a single system, the use of many separate setsof housing, electronics and power supplies would result in a system thatis complex, heavy and bulky. In addition, such integration approachwould require using multiple telescopes, one for every laser in thesystem, or the combining of outputs of all lasers into a single beam, tobe transmitted through a single telescope. Both of these methods wouldrequire careful bore-sighting of all of the beams relative to eachother, and maintaining such alignment over a large military-spectemperature range and vibration and shock exposure. The need for suchbeam combiner and/or multiple telescopes would further add to thecomplexity, weight and size of the multi-wavelength laser system. Thesedeficiencies would make a multi-wavelength laser system that iscomprised of separate individual lasers too large and complex to beuseful. This is particularly true when a laser system needs to behand-held, rifle-mounted or be mounted in a small unmanned air vehicles(UAVs) and unmanned ground vehicles (GUVs).

U.S. Pat. No. 5,675,595 relates to a composite multiple wavelength lasermaterial and multiple wavelength laser for use therewith; U.S. Pat. No.4,494,235 relates to a multiple wavelength laser; and U.S. Pat. No.5,331,649 relates to a multiple wavelength laser system. These patentsdo not teach generating multiple beams with different wavelengths, pulseenergies, pulse repetition rates. Further, they are not known to teachcontrol of the output energy and power of each laser independently.

SUMMARY OF THE INVENTION

In one aspect, a compact multi-wavelength and multi-beam laser isdisclosed. Such a multi-wavelength and multi-beam laser comprises anoptically segmented composite gain medium; a single common laserresonator defined by a first common surface mirror and a second commonsurface mirror, at least one of said common surface mirrors beingconfigured as a common surface segmented mirror; a segmented laser diodepump; and a set of intra-cavity optical components uniquely configuredper each laser beam of said multi-beam laser arranged within the singlelaser resonator.

In another aspect, an optically segmented composite gain medium isdisclosed. Each segment of the composite gain medium functions as anoptical waveguide for a respective pump light of a segmented laser diodepump array. Such an optically segmented composite gain medium comprisesa plurality of gain medium sections, each gain medium section beingbased on a selected gain material to function as an optical waveguidefor the respective pump light; and separation regions defined by aregion separating adjacent regions of said plurality of gain mediumsections for optical isolation based on a material of lower refractiveindex. The lower index material forms each separation region.

Yet, in another aspect, a method of forming a single common laserresonator cavity is disclosed based on the optically segmented compositegain medium. Such a method comprises disposing two common surfacemirrors that are based on an optically flat reflector such that a commonsurface mirror is commonly shared by all of the laser segments formed bythe separate gain segments of the optically segmented composite gainmedium and other optical components arranged in each laser segment; andangle-aligning said two common surface mirrors relative to each other toform the single common laser resonator cavity, an optical propagationaxis through the resonator cavity being defined perpendicular to themirrors. Each of the mirrors are flat within 1/20 of optical wavelengthacross the entire mirror surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages and features will become apparent as the subjectinvention becomes better understood by reference to the followingdetailed description when considered in conjunction with theaccompanying drawings wherein:

FIG. 1 shows an exemplary embodiment of a multi-wavelength, multi-beamlaser, depicted as an exemplary side-view configuration, with arespective corresponding frontal view shown below selected elements.

FIG. 2 shows exemplary gain medium with optically segmented, compositestructure.

FIG. 3 a shows an exemplary common laser resonator cavity defined by twocommon surface mirrors on separate substrates.

FIG. 3 b shows an alternate exemplary common laser resonator cavitydefined by two common surface mirrors, the first mirror having beendeposited directly on the end facet of a multi-segment gain structure.

FIG. 4 shows an exemplary common surface laser resonator mirror havingseparate sections or segments.

FIG. 5 shows an exemplary segmented laser diode pump array, shown as a4-section array.

DETAILED DESCRIPTION

An exemplary embodiment of a single laser is disclosed that generatesemission in multiple beams, multiple wavelengths and temporalcharacteristics required for the various applications enumerated above.All of the beams are generated in a single laser resonant cavity and areinherently bore-sighted relative to each other, removing the need toindividually align the output angles of the separate beams. The relativebore-sighting of the individual beams also makes it possible to use asingle telescope to transmit all of the laser beams, without the use ofbeam combiner. Other aspects of the exemplary laser embodiment are thateach of the multiple laser beams can be at a vastly different (from UVto mid-infrared) wavelength, pulse energy, pulse length, repetition rateand average power. The power and/or pulse energy of each laser beam canbe separately controlled and turned on or off independently.Particularly, for some applications, such as eye-safe range-finding at1550 nm, residual emission from the other lasers operating at otherwavelengths could preclude eye-safe operation of the range-finder. Suchcharacteristics of the disclosed multi-wavelength and multi-beam lasersystem are made possible by using an exemplary laser embodimentcomprised of: an optically segmented gain section, an opticallysegmented pump laser, and common surface segmented cavity mirrors.

The configuration of such an exemplary embodiment of a multi-wavelength,multi-beam laser is shown in FIG. 1. FIG. 1 shows an exemplary side-viewconfiguration of the pertinent elements, with a respective correspondingfrontal view shown below selected elements. The disclosed exemplarymulti-wavelength, multi-beam laser embodiment 100 is comprised of thefollowing elements:

-   1. Optically segmented, composite gain medium (e.g., based on    Nd:YAG) 110.-   2. A single common laser resonator defined by common surface    mirrors, at least one of the first common surface mirror 120 and the    second common surface mirror 130 being configured as common surface    segmented mirrors, a common surface segmented mirror (120 and/or    130) having segments (e.g., 131-134) with each segment having    different reflectivity, both in magnitude and spectral dependence.-   3. A segmented laser diode pump array 140, comprising individual    electrically isolated segments that can be activated separately, and    with each section (e.g., 141-144) selected to have a specific    emission wavelength optimized for pumping the corresponding segment    in the gain medium.-   4. Multiple lasers arranged within the single laser resonator, each    laser configured with a selected combination of intra-cavity optical    components, e.g., Cr:YAG, non-linear crystal, polarizer,    electro-optic Q-switches 151-154, optimized to generate a specific    laser emission wavelength, pulse energy, average power, and pulse    repetition frequency.

These key parts and their function will now be described.

1. Optically Segmented Gain Medium.

An exemplary gain medium 200 with optically segmented, compositestructure is shown in FIG. 2. It is comprised of several gain mediumsections (e.g., 210, 220, 230 and 240), separated by material of lowerrefractive index, the lower index material forming aseparation/isolation region 250. The refractive index step between thegain material and the lower refractive index region causes all lightbelow the critical angle to be reflected at the boundary. This allowseach gain section (e.g., 210, 220, 230, 240) to function as an opticalwaveguide for the pump light (e.g., 201); pump light injected into eachsection is wave-guided along the section (e.g., 210, 220, 230 or 240),without coupling into the adjoining gain sections, isolating the pumplight in that section from that in the other sections. The divergenceangle of the pump light propagating in the waveguide is assumed to besmaller than the critical reflection angle defined by the refractiveindex difference between the gain medium (e.g., 210, 220, 230 or 240)and the lower refractive index material 250.

As an example, the composite structure (e.g., 200) can be made of slabsor rods of Nd:YAG, Yb:YAG, Nd:YVO, Nd:YLF, Er/Yb doped glass, Er:YAG,separated by lower index epoxy or bonding glass, resulting in aquasi-monolithic composite gain structure. With this approach, eachsection (e.g., 210, 220, 230 or 240) can be made of a different gainmaterial. The multi-segment gain structure can also be made using amonolithic Nd:YAG ceramic material in which the higher index Nd-dopedsections are surrounded by lower index un-doped YAG. Alternatively,another exemplary embodiment of a monolithic structure (e.g., 200) couldbe Er/Yb doped phosphate-silica glass rods surrounded by lowerrefractive index silica glass. The end faces of both thequasi-monolithic and monolithic composite structures can be end-polishedand optically coated as a single piece, simplifying the assembly. Inaddition, this technique will result in end-facets that are opticallyflat across all of the gain sections, so that a common surface lasercavity mirror (as explained below) can be directly deposited on thecomposite gain structure. In another aspect, an exemplary multi-segmentgain medium structure (e.g., 200) can also be fabricated by attachingthe individual rods to each other or surrounding mechanical supportusing by small spacers and allowing the remaining surfaces to besurrounded by air. The small spacers are preferably made out of lowindex material to prevent pump light leakage out of each gain section,or are sufficiently short that the leakage is negligibly small.

2. A Method for Forming A Single Common Laser Resonator

An exemplary embodiment of a single common laser resonator cavity 300 isdefined by two common surface mirrors (e.g., 320, 330), as shown inFIGS. 3 a and 3 b.

The exemplary laser resonator is formed by two flat mirrors (e.g., 320,330 for FIG. 3 a), precisely angle-aligned relative to each other toform an optical resonator. An optical propagation axis for the laserlight inside the resonator cavity is defined by the mirrors (e.g., 320,330), and is perpendicular to them. The “common surface” terminologyrefers to the fact that both mirrors (e.g., 320, 330) constitute anoptically flat reflector that is common or shared by all of the lasersegments formed by the separate gain segments (e.g., 311-314) and otheroptical components (e.g., combinations of intracavity optical components351, 352, 353 and/or 354 of 350 as shown) arranged in each lasersegment. The mirrors are typically flat within 1/20 of opticalwavelength across the entire mirror surface. Since the flat mirror laserresonator 300 is common to all laser segments (e.g., 311-314), all ofthe laser beams propagate along the same optical axis in the laser andexit the laser cavity at same angle, ensuring that all of the multiplelaser output beams are bore-sighted relative to each other. One of thecommon surface mirrors (first mirror in FIG. 3 a or 3 b) can be on aseparate substrate 320 as shown in FIG. 3 a, or alternatively, at leastthe first mirror can be deposited directly on the end facet 321 of amonolithic or semi-monolithic multi-segment gain structure 310, as shownin FIG. 3 b.

3. Common Surface Segmented Mirror

One or both of the first and second common surface laser resonatormirrors can have separate sections or segments, as shown in FIG. 4,where such a mirror 400 is comprised of, e.g., four separate segments(410, 420, 430, 440).

The reflectivity and spectral characteristics of each mirror segment(410, 420, 430 or 440) can be individually optimized for eachcorresponding laser segment of the multi-wavelength laser. All of themirror segments (e.g., 410, 420, 430, 440) are deposited on an opticallyflat surface of the mirror substrate (e.g., 400), so reflections fromall segments will be co-aligned in angle. This feature assures that alaser resonant cavity will be optimally aligned for all of the laserssimultaneously. For a typical laser resonator defined by two flatmirrors this requires a mirror angular alignment accuracy ofapproximately 100 micro-radians, a precision that can be achieved withwell established laser fabrication procedures.

4. A Segmented Laser Diode Pump Array

An exemplary embodiment of a segmented laser diode pump array 500 iscomprised of several electrically isolated pump sections (e.g., 510,520, 530, 540) that can be activated individually. Each section in thesegmented laser diode array, such as a 4-section array 500 shown in FIG.5, consists of a diode bar or a multi-bar stack, with all laser diodesmounted on a single common heat-sink. All diode bars or diode stacks inan array section (510, 520, 530 or 540) are electrically isolated fromthose in other sections. This feature makes it possible to control thepower output from each pump array section individually by controllingcurrents I₁, I₂, I₃, I₄ flowing through each of the four sections (510,520, 530, 540), respectively. By this means each corresponding lasersegment in the multi-wavelength laser of FIG. 3 a or 3 b can beactivated separately.

If only one pump array segment is activated, the other pump arraysections are in off, so that the corresponding laser segments of themulti-wavelength laser are not activated and do not generate any laseremission. If the various gain section are made of different gain medium,each section might require different pump wavelength. To accommodatethis need, each segment (e.g., 510, 520, 530, 540) in the pump array 500can be made with different semiconductor laser diode composition, sothat each the emission wavelength of each is optimum for thecorresponding laser segment in the multi-wavelength laser.

5. Multiple Lasers Arranged within the Single Laser Resonator

Multiple laser segments are arranged within the multi-wavelength singlelaser resonator, as shown, e.g., in FIGS. 3 a and 3 b. Each lasersegment (e.g., 311, 312, 313 or 314) can be configured with a selectedcombination of: pump laser wavelength and power, gain medium, commonsurface mirror reflectivity, intra-cavity optical elements (e.g.,351-354). The specific combination of these components (an exemplaryconfiguration of intra-cavity optical elements being referenced as 350)is chosen to generate a desired laser emission wavelength, pulse energy,average power, and temporal characteristics of pulse repletion frequencyand pulse length. The intra-cavity optical elements comprising anexemplary configuration of intra-cavity optical elements 350 caninclude, but are not limited to: a Q-switch, which can be either asaturable absorber such as Cr:YAG for passive Q-switching, or anelectro-optical crystal such as LiNbO₃ for active Q-switching; apolarizer for defining the polarization state of the laser emission; anon-linear crystal for frequency conversion, such as a KTP crystal forfrequency doubling of 1064 nm emission, or a KTP crystal for a OpticalParametric Oscillator (OPO) for converting 1064 nm emission to 1560 nmeye-safe emission wavelength or 3.3 μm mid-IR emission; spectrallyselective filters to define the laser emission wavelength; apertures toselect spatial modes of the laser.

It is obvious that many modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as described.

What is claimed is:
 1. An optically segmented composite gain medium,each segment of the composite gain medium functioning as an opticalwaveguide for a respective pump light of a segmented laser diode pumparray, said optically segmented composite gain medium comprising: aplurality of gain medium sections, each gain medium section being basedon a selected gain material to function as an optical waveguide for therespective pump light; and separation regions defined by a regionseparating adjacent regions of said plurality of gain medium sectionsfor optical isolation based on a material of lower refractive index, thelower index material forming each separation region.
 2. The opticallysegmented composite gain medium recited in claim 1, wherein a refractiveindex step between the respective adjacent gain material and the lowerrefractive index region causes all light below the critical angle to bereflected at the respective boundary, allowing each gain medium sectionto function as an optical waveguide for the respective pump light, pumplight injected into each section being wave-guided along the respectivesection without coupling into adjoining gain sections.
 3. The opticallysegmented composite gain medium recited in claim 1, wherein each sectionof gain medium is either a slab or a rod based on a selection ofmaterial chosen from a group consisting of Nd:YAG, Yb:YAG, Nd:YVO,Nd:YLF, Er/Yb doped glass and Er:YAG, each slab or rod being separatedby a lower index epoxy or bonding glass, resulting in a quasi-monolithiccomposite gain structure.
 4. The optically segmented composite gainmedium recited in claim 1, wherein said gain medium sections are basedon a monolithic Nd:YAG ceramic material, and the separation regions arebased on a lower index un-doped YAG, wherein the higher index Nd-dopedsections are surrounded by lower index un-doped YAG.
 5. The opticallysegmented composite gain medium recited in claim 1, wherein said gainmedium sections are based on Er/Yb doped phosphate-silica glass rods,and the separation regions are based on a lower refractive index silicaglass, wherein said Er/Yb doped phosphate-silica glass rods aresurrounded by said lower refractive index silica glass.
 6. The opticallysegmented composite gain medium recited in claim 1, wherein end faces ofsaid gain medium sections are end-polished and optically coated as asingle piece, and wherein a common surface laser cavity mirror isdirectly deposited on at least one of the end faces.
 7. The opticallysegmented composite gain medium recited in claim 1, wherein saidseparation regions are defined by small spacers based on a low indexmaterial to prevent pump light leakage out of each gain section.
 8. Theoptically segmented composite gain medium recited in claim 1, whereinsaid segmented laser diode pump array is comprised of severalelectrically isolated pump sections that can be activated individually,each section in the segmented laser diode array being based on a diodebar or a multi-bar stack, with all laser diodes mounted on a singlecommon heat-sink.